In the railway industry it has been common practice to support the opposed ends of a freight car body on spaced-apart, wheeled truck assemblies for movement along a railway track. The standard truck assembly is the three-piece truck, so called because its principal structural members are a pair of elongated side frames which extend generally longitudinally of the railway track between a pair of spaced wheelsets, and an elongated bolster which extends transversely of the track and has its opposed ends supported by the respective side frames.
In a conventional truck, each longitudinal end of the bolster is received in a respective opening or window formed intermediate the longitudinal ends of the respective side frame and is supported therein by a suspension system including a spring set which permits movement of the bolster relative to the side frame. The springs typically extend between a spring seat in the side frame opening and a respective undersurface of the bolster end which is spaced above the side frame spring seat. The bolster ends thus are supported for movement within predetermined ranges of lateral and vertical motion. Lateral deflection of the springs is limited by pairs of bolster stops or gibs disposed to engage the laterally opposed sides of the two spaced columns of each side frame, and vertically downward spring deflection is limited by the deflection at which the spring coils are in solid, vertically abutting engagement.
Other modes of relative motion between the bolster and the side frames in conventional three-piece trucks include relative bolster rotation about axes extending longitudinally of the bolster or longitudinally of the side frames, as well as about axes which are perpendicular to both of these. For example, movement of the truck to an out-of-square or warped configuration involves relative rotation of the bolster with respect to the side frames about a vertical axis.
The railway track on which frieght cars run often is non-uniform due to such causes as differential settling that results from non-uniform ballast or foundation under the railway ties, non-uniform rail wear and rail misalignment. As a result, vertical and lateral track variations can impart energy to the truck suspension system and in turn cause the car body to rock, bounce or sway. With commonly used spring suspension systems, a railway car body has a natural resonant frequency of sway and bounce. If the track conditions are such as to cause the car body to sway or bounce at its resonant frequency, the resulting car body motion can force the suspension against its lateral and vertical mechanical limits, characterized above as solid springs vertically and rigid bolster gibs laterally.
In a conventional truck the rigid bolster gibs confront laterally opposed reaction surfaces disposed on laterally opposite sides of the respective side frame columns adjacent sidewall portions of the side frame window within which a bolster end is received. The space between the opposed gibs exceeds the spacing between the respective side frame reaction surfaces by a given dimension which determines the magnitude of available bolster movement in the lateral direction, i.e., transversely of the tracks. Similarly, the geometry and dimensioning of the conventional gibs defines the limits of such other modes of relative motion as rotation of the bolster with respect to the side frames about vertical axes, and rocking of the side frame about its longitudinal axis.
In one of the most common expendients presently being employed to control car body rock and sway, the energy input to the suspension systems of rail car trucks is dissipated by use of bolster friction elements which utilize rigid (commonly metallic) friction wedges or friction shoes to damp relative motion between the truck bolster and the side frames. Elastomeric friction shoes, as well as combined elastomeric and rigid friction shoe assemblies are also known. Nearly all freight car trucks built within the past 40 years have included bolsters with opposed pockets formed in the longitudinal ends thereof that receive such friction elements for the purpose of damping or dissipating the kinetic energy of relative bolster-to-side frame motion.
Conventional friction assemblies have provided not only frictional energy dissipation to damp relative movement between the bolster and the side frames, but also bolster-to-side frame fit up and a degree of control over certain modes of relative motion to which the bolster and side frame assembly may be subjected.
In addition to rigid or elastomeric friction elements, hydraulic snubbers are also used to dissipate the energy input to railway car trucks; however, hydraulic snubbers do not rely upon dry or coulomb friction to dissipate energy. Such snubbers are mentioned here only to emphasize that control of relative bolster and side frame motion requires control of the force inputs to the railway truck by conversion of sufficient kinetic energy into heat energy, and thereafter dissipation of the heat energy to the atmosphere.
Practitioners in the art have continually sought to develop improved structures for controlling and limiting relative motion between the truck side frames and bolster, including but not limited to various arrangements of friction elements with angled or inclined biasing surfaces that are engageable with cooperably angled column guide surfaces of the side frames to provide improved modes of control over relative bolster-to-side frame motion, for example, as disclosed in U.S. Pat. No. Re. 31,988.
Other patent art known to the Applicant herein which relates generally to friction elements for railway truck bolster-to-side frame fit up includes the following, all generally pertaining to double friction shoe arrangements in the opposed bolster pockets, or contoured bolster pockets: U.S. Pat. Nos. 2,434,838, 2,458,210, 3,687,086, 2,257,109, 2,408,866, 2,424,936, 2,456,635, 2,528,473, 2,570,159, and 3,026,819 all relate to vertically split two-piece friction wedges; U.S. Pat. Nos. 2,324,267, 2,367,510, 2,650,550, 2,661,702, 2,688,938, and 2,853,958 all pertain to contoured column wear plate configurations such as sloped or tapered friction element contact surfaces; U.S. Pat. Nos. 4,179,995 and 3,670,660 pertain to railway truck bolster gib arrangements other than the commonly encountered bolster gibs which are engageable with laterally opposed contact surfaces of the respective side frames.